E. Nichelatti

Soft x-ray submicron imaging detector based on point defects in LiF

The use of lithium fluoride (LiF) crystals and films as imaging detectors for EUV and soft-x-ray radiation is discussed. The EUV or soft-x-ray radiation can generate stable color centers, emitting in the visible spectral range an intense fluorescence from the exposed areas. The high dynamic response of the material to the received dose and the atomic scale of the color centers make this detector extremely interesting for imaging at a spatial resolution which can be much smaller than the light wavelength. Experimental results of contact microscopy imaging of test meshes demonstrate a resolution of the order of 400nm. This high spatial resolution has been obtained in a wide field of view, up to several mm2. Images obtained on different biological samples, as well as an investigation of a soft x-ray laser beam are presented. The behavior of the generated color centers density as a function of the deposited x-ray dose and the advantages of this new diagnostic technique for both coherent and noncoherent EUV so...

Complex refractive index of a slab from reflectance and transmittance: analytical solution

Analytical expressions are derived that allow one to calculate the complex refractive index of a planar slab from normal-incidence intensity reflectance and transmittance.

Optical characterization of low-energy electron-beam-colored LiF crystals by spectral transmittance measurements

The complex refractive index of a LiF crystal surface layer irradiated by low-energy electrons is modified by the stable formation of color centers embedded in it. A simplified dipole-electromagnetic field interaction model has been adopted in order to estimate the dispersion curves of colored LiF from a single optical transmittance measurement. The excellent agreement with the corresponding experimental curves (obtained by means of spectrophotometry and ellipsometry) demonstrates this to be a promising approach for LiF-based optical waveguide characterization.

High-contrast photoluminescent patterns in lithium fluoride crystals produced by soft x-rays from a laser-plasma source

A technique using soft x-rays and extreme ultraviolet light generated by a laser-plasma source has been investigated for producing low-dimensionality photoluminescent patterns based on active color centers in lithium fluoride (LiF) crystals. Strong visible photoluminescence at room temperature has been observed in LiF crystals from fluorescent patterns obtained by masking the incoming radiation. This technique is able to produce colored patterns with high spatial resolution on large areas and in short exposure times as compared with other coloration methods.

Characterisation of antireflective TiO2//SiO2 coatings by complementary techniques

Abstract Reactive magnetron sputtering deposition processes based on rotating or double cathodes allow the deposition of multiple TiO2//SiO2 layers on glass for obtaining antireflective coatings for large area applications with a good homogeneity and high resistance against environmental attacks. By increasing the number of TiO2//SiO2 layers, the luminous reflectance of the coated glass decreases but the calculation of angular daylight and energy parameters requires the use of an increasing number of complementary analytical techniques due to the complexity of the coating. In this paper we show how Rutherford back-scattering, neutron reflectivity and spectrophotometry can be successfully utilised to perform a quite complete characterisation of commercial antireflective coatings based on three TiO2//SiO2 layers.

Reflectance and transmittance of a slightly inhomogeneous thin film bounded by rough, unparallel interfaces

Inhomogeneity of a thin film, rough interfaces of the film with air and substrate and non-parallelism of these interfaces severely affect reflectance and transmittance. Neglecting these film imperfections could give rise to erroneous optical characterisation of the film from measured spectra. Each one of these imperfections, considered alone, has its own characteristic influence on the interference fringes of the reflectance and transmittance spectra. In this work, a model of an inhomogeneous thin film bounded by rough, unparallel interfaces is introduced for calculating reflectance and transmittance at normal incidence. The effects on the spectra due to roughness as predicted by the proposed model are compared with those evaluated by an approach involving the effective-medium approximation (EMA). In this latter approach, a thin transition layer replaces the rough interface, but it proves inadequate for reproducing the transmittance spectrum. Finally, the model is successfully applied to the characterisation of a LiF film thermally evaporated on a glass substrate.

Mode analysis in He+-implanted lithium fluoride planar waveguides

The depth refractive index profiles of broadband visible-emitting planar waveguides produced in LiF crystals with 1.5- and 2-MeV He+ ions at different doses have been derived from mode analysis. They show that there are two competitive mechanisms responsible for positive and negative modifications of the refractive index in the irradiated volume associated with different processes of energy deposition of the incident ions, so as to induce a complex coloration profile along the penetration direction, which is strongly dependent on the irradiation dose.

Thermoluminescence in pure LiF crystals: Glow peaks and their connection with color centers

Nominally pure LiF crystals were irradiated with the same dose (0.85 106 R) of gamma rays at ambient and low temperatures (−60 °C) and the resulting thermoluminescence (TL) is reported. Various optical and thermal treatments were applied in order to change the concentration of color centers (CCs). The effect of such treatments on the glow curves is observed. Knowing the coloration from optical transmission and photoluminescence measurements made on the same samples, we attribute many of the glow peaks (GPs) to the annealing of F center aggregates. For the present conditions of irradiation and dose, TL processes begin with decay of F3+ centers that display a GP at 164 °C. F3(R) centers follow and are responsible for GPs at 193 and 228 °C. A GP at 263 °C is ascribed to F2 centers. Several peaks at temperatures in the range of 280–380 °C are associated with impurity perturbed F centers. A GP at 410 °C is associated with a complex of aggregated F and H centers. These attributions are accomplished by means of ...

Spatial and spectral response of a Fabry–Perot interferometer illuminated by a Gaussian beam

A generalized study has been done of the transmission characteristics of a Fabry-Perot interferometer (FPI) illuminated by a Gaussian light beam impinging on it at normal and non-normal incidence. The theoretical approach is based on a plane-wave, angular-spectrum representation of both the incident Gaussian beam and the transmitted beam. Expressions are obtained for the FPI instrumental function and for the spatial distribution of the transmitted beam. Numerical results are presented for the FPI maximum transmission, effective finesse, and spectral displacement of the interference maximum.

Color-Center Waveguides in Low-Energy Electron-Bombarded Lithium Fluoride

We employ a differential version of m-line spectroscopy through grating coupling in order to measure the refractive index of low-energy electron irradiated lithium fluoride crystal channel waveguides for broadband emission. Using photoresist films and a holographic setup for grating fabrication, we perform an accurate characterization of bidimensional structures and a direct quantitative comparison between treated and blank materials.